Code-division multiple access (“CDMA”) wireless communications systems offer improved capacity and reliable communications. Capacity of a cellular system is important to cellular service providers because it directly impacts revenue. In general, the capacity of a CDMA wireless communications system is limited by interference. Therefore, it is beneficial to minimize the amount of interference in a CDMA wireless communications system.
Generally, the most significant amount of interference is generated from other mobile units, such as cellular phones, operating within the cell or from neighboring cells. On the reverse link, i.e. transmission from a mobile unit to a base station, a mobile unit in a CDMA wireless communications systems transmits a pseudo-noise (“PN”) sequences to the base station. The base station receives this signal as well as other mobile units' transmitted PN sequence. PN sequences have the property that correlation between delayed versions of one sequence are much lower than that between two sequences that are time-aligned, that is, a delayed PN sequence appears as noise to the receiver time-aligned to receive a second PN sequence with a different delay. Thus, a number of mobile units can transmit on the same frequency to the same base station in a CDMA system. A CDMA mobile user's transmitted signal contributes to interference to the transmitted signal of all other users.
Because the signal of each mobile unit interferes with the signals of other mobile units in a typical CDMA cellular environment, an interference problem exists known as the “near-far” problem. To illustrate the near-far problem, consider the case of two mobile units operating in communication with the same base station. Suppose that a first mobile unit is near the base station and has a small path loss and a second mobile unit is far from the base station and has a large path loss. Also, assume that the two mobile units transmit using the same amount of power. Since the two mobile units transmit with the same amount of power but have different amounts of path losses, the base station may receive a weaker signal from the second mobile than the signal from the first mobile unit. In a CDMA system, the transmitted signal of each mobile unit adds interference to all other mobile units. It can be seen then that the first mobile unit becomes a relatively larger interference source to the second mobile unit than the second mobile unit is to the first, as seen from the base station. Thus, a mobile unit close to the base station drowns out the signal of a mobile unit far from the base station. To overcome the near-far problem, CDMA wireless communications systems use power control to control the transmitted power of each mobile unit. In general, CDMA wireless communications systems use three types of power control for the reverse link, open-loop power control, closed-loop power control, and outer-loop power control. In the following, terminology from the IS-2000 standard is used as an example. In open-loop power control, a mobile unit uses the estimated received power from the base station to control its transmitted power. Typically, open-loop power control by itself is insufficient because the forward link (i.e., base station to mobile) and reverse link (i.e., mobile unit to base station) utilize different frequency bands. As such, the shadowing and fading characteristics for the forward link and the reverse link can be different. Thus, CDMA wireless communications systems also use (a) closed-loop power control, which adjusts the transmitted power of a mobile unit so that its received signal to noise and interference ratio at the base station is as close to a desired level as possible; and (b) outer-loop power control which determines what the desired signal to noise and interference ratio is.
In concept, closed-loop power control seeks to adjust the transmitted power of a mobile unit so that its transmitted signal received at the base station is as close to a threshold value as possible. At the base station, the closed-loop power control sends an up/down command to a mobile unit if the closed-loop power control determines that the mobile unit needs to increase or decrease its transmitted power. The closed-loop power control uses the output of the outer-loop power control, i.e. a desired signal to noise and interference ratio called the set point, as the threshold to determine if the mobile units received signal at the base station is too high or too low.
The outer-loop power control is implemented at the base station so that the frame erasure rate (“FER”) target or other quality metric is achieved with minimal transmitted power for the channel under control. If the link quality is too low or too high, the base station adjusts the outer-loop set point up or down to achieve the desirable link quality. The adjustment of the outer-loop set point is outer-loop power control.
CDMA wireless communications standards such as IS-2000 offer higher data rates than those of older CDMA standards. In the reverse link of IS-2000, a mobile unit can transmit at higher data rates using one or more Reverse Supplemental Channels (“R-SCHs”) in addition to the Reverse Fundamental Channel (“R-FCHs”), which typically is used for lower data rates. R-SCHs operate at different received signal to noise and interference levels from those of the R-FCH.
At lower rates, in general, the mobile unit transmits either on the R-FCH or the Reverse Dedicated Control Channel (“R-DCCH”). The base station observes the FER of the R-FCH or the R-DCCH and adjusts the outer-loop set point based on that FER. When the mobile unit transmits at a higher data rate, it transmits on R-SCH in addition to R-FCH, R-DCCH, or both.
As mentioned previously, the R-SCH in general operates at different received signal to noise and interference levels from that of the R-FCH or R-DCCH. This, in turn, affects the base station's optimal level of the received signal to noise and interference ratio for the R-Reverse Pilot Channel (“R-PICH”). The base station uses a different outer-loop set point on the R-PICH received signal to noise and interference ratio when the mobile transmits on the R-SCH. In order to adjust the outer-loop set point when R-SCH is used, one method is for the base station to observe the FER or other decoder metrics of the R-SCH and use that in the outer-loop to adjust the outer-loop set point
However, there are several problems with observing the FER of the R-SCH to adjust the outer loop set point. One problem is that the mobile unit, in general, is only allowed to transmit on the R-SCH for a limited duration. That limited duration does not provide enough observation time to generate meaningful FER statistics that are necessary for fine-tuning the outer-loop set point. Another problem is that the transmission of R-SCH can be abruptly terminated by the mobile unit. For example, the mobile unit may not have adequate amount of RF power or any more data to transmit on the R-SCH(s). As a result of this unscheduled termination of transmission on the R-SCH(s), the estimate of the FER becomes difficult at the base station. Even when the termination of the R-SCH transmission occurs according to a pre-arranged schedule, the outer-loop power control has to switch back and forth between the R-SCH and the R-FCH or the R-DCCH. If the outer-loop on either R-FCH or R-DCCH has not been updated, such transitions can create a period of settlement where an otherwise unnecessary large transmit power would be needed to maintain the link quality on R-FCH or R-DCCH. Similar loss of efficiency happens when the outer-loop transitions to the decoder(s) of the R-SCH(s).
In parallel with the outer-loop set point for R-SCH, similar problems can exist for the R-DCCH due to its bursty transmissions. That is, the outer-loop power set point might not settle at the right level if frequent transmissions do not occur in the underlying channel. When only the R-DCCH is used in addition to the R-PICH on the reverse link of a mobile unit, there is a need to raise the received pilot to noise and interference ratio to compensate for the fact that the outer-loop set point is not subject to frequent updates. Since the IS-2000 standard defines a fixed traffic to pilot ratio for both R-FCH and R-DCCH regardless of their likely transmission duty cycles, the need to adjust the pilot reference ratio only for the R-DCCH must be resolved.
Unless novel techniques are used for the outer-loop power control when a mobile unit transmits using a plurality of channels, a mobile unit may transmit more power than needed for the desired link reliability. This, in turn, may reduce battery life for a mobile unit and decrease the reverse link capacity of the cellular system. There is therefore need in the art for outer-loop power control when a mobile unit transmits using multiple channels. Also, it is desirable to have a low complexity solution in resolving the outer-loop power control problem, especially in fast time-to-market conditions. A simple solution reduces the tuning needed, lowers the possibility of mistakes in implementation, and increases the robustness of the system under unexpected operating conditions. In addition, a solution that can be implemented with minimal changes to hardware and software reduces the design time, which consequently reduces engineering costs.